This invention relates to a turbofan driven emergency generator, which is driven by the turbofan when the turbine engine loses power.
In modern, turbofan power aircraft, an emergency power source is required for control of flight surfaces in the event of the total loss of the availability of the primary power sources; i.e. engine driven hydraulic pumps and/or engine driven electrical generators. For small airplanes, this power is provided by the energy stored in aircraft batteries. For larger airplanes, a single ram air turbine (RAT) with an integral generator or hydraulic pump is provided for deployment in an emergency situation only. Here, the emergency power source is the aircraft's own airspeed (kinetic energy) and altitude (potential energy).
To derive maximum power from the ram air, a RAT must be located away from aircraft surfaces that would disturb the smooth (laminar) entry of air into the RAT blades. In practice, this usually means mounting the RAT under the wing or below the nose of the aircraft. Finding a suitable location for the RAT and designing a deployment system to position the RAT into the airstream can present significant challenges for the aircraft design.
An additional consideration is that, there is a possibility that the unshrouded RAT blades could separate from their hub at high velocity during RAT operation, presenting a containment issue.
Also, to minimize installation envelope, RAT systems are typically designed such that once that RAT is deployed it cannot be retracted in flight. For this reason the RAT system is one of very few aircraft systems that is not routinely tested in pre-flight or flight. Scheduled ground testing to deploy and back-driving the RAT must be performed to ensure it will function properly in an emergency event. Again, unshrouded turbine blades are undesirable.
In view of the above, a turbine engine has been devised that employs one or more generators that is driven by the turbofan in a windmill condition when the engine has lost power. However, the generators are continuously coupled to the turbofan and driven at the same speed as the turbofan. As a result, designing a generator that can operate under windmill conditions and high speeds during normal turbine engine operation is problematic. Either too much voltage is produced at high speeds or the generator is too small for windmill conditions. Said another way, the difference in speeds between engine windmilling and engine operating modes may amount to as much as an 8:1 overall speed ratio. This presents a difficult design challenge in that the generator must be large enough to provide full rated power at the lowest windmilling speed and strong enough to maintain mechanical integrity at the highest engine speed.